📚 Respiration without Oxygen | 无氧呼吸
When oxygen is unavailable, cells cannot use the electron transport chain, so they must regenerate NAD⁺ by alternative pathways. This article explains glycolysis, lactate fermentation in animals, alcoholic fermentation in yeast, energy yields, oxygen debt, and common exam pitfalls for Cambridge A-Level Biology.
当氧气不可用时，细胞无法利用电子传递链，因此必须通过替代途径再生 NAD⁺。本文讲解糖酵解、动物乳酸发酵、酵母酒精发酵、能量产出、氧债以及剑桥 A-Level 生物学常见考试误区。
1. Why Oxygen Matters in Aerobic Respiration | 为什么有氧呼吸需要氧气
In aerobic respiration, oxygen acts as the final electron acceptor at the end of the mitochondrial electron transport chain. Without oxygen, electrons cannot be removed from the chain, proton pumping stops, and ATP synthase cannot generate ATP by chemiosmosis.
在有氧呼吸中，氧气作为线粒体电子传递链末端的最终电子受体。没有氧气，电子无法从传递链中被移除，质子泵停止工作，ATP 合酶无法通过化学渗透产生 ATP。
This also means that reduced coenzymes such as NADH and FADH₂ cannot be reoxidised to NAD⁺ and FAD. Because glycolysis needs a constant supply of NAD⁺, anaerobic cells must use fermentation to recycle it.
这也意味着还原型辅酶如 NADH 和 FADH₂ 无法被重新氧化为 NAD⁺ 和 FAD。由于糖酵解需要持续供应 NAD⁺，厌氧细胞必须利用发酵来循环它。
2. The Universal First Stage: Glycolysis | 共同的第一阶段：糖酵解
Glycolysis takes place in the cytoplasm and does not require oxygen. It splits one glucose molecule (6C) into two molecules of pyruvate (3C). The process involves phosphorylation using 2 ATP, lysis of the hexose sugar, oxidation to produce 2 reduced NAD, and dephosphorylation to produce 4 ATP.
糖酵解发生在细胞质中，不需要氧气。它将一个葡萄糖分子（6C）分解为两个丙酮酸分子（3C）。该过程包括消耗 2 个 ATP 进行磷酸化、己糖裂解、氧化产生 2 个还原型 NAD，以及去磷酸化产生 4 个 ATP。
C₆H₁₂O₆ + 2 NAD⁺ + 2 ADP + 2 Pᵢ → 2 CH₃COCOO⁻ + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O
The net gain is therefore 2 ATP per glucose by substrate-level phosphorylation. In aerobic conditions, pyruvate enters the link reaction and Krebs cycle; without oxygen, it is diverted into fermentation.
因此，每分子葡萄糖通过底物水平磷酸化净产生 2 个 ATP。在有氧条件下，丙酮酸进入连接反应和克雷布斯循环；没有氧气时，它被转入发酵途径。
3. The Problem: NAD⁺ Must Be Recycled | 关键问题：NAD⁺ 必须再生
NAD⁺ is only present in cells in small amounts. Glycolysis oxidises glyceraldehyde 3-phosphate and reduces NAD⁺ to NADH. If NADH accumulates and NAD⁺ runs out, glycolysis stops, and cells lose their only anaerobic source of ATP.
细胞中 NAD⁺ 的含量很少。糖酵解氧化甘油醛-3-磷酸并将 NAD⁺ 还原为 NADH。如果 NADH 积累而 NAD⁺ 耗尽，糖酵解就会停止，细胞就会失去唯一的无氧 ATP 来源。
In aerobic respiration, the electron transport chain reoxidises NADH. In anaerobic conditions, fermentation pathways transfer electrons from NADH to an organic molecule such as pyruvate or acetaldehyde, regenerating NAD⁺ without making ATP directly.
在有氧呼吸中，电子传递链重新氧化 NADH。在无氧条件下，发酵途径将电子从 NADH 转移给丙酮酸或乙醛等有机分子，再生 NAD⁺ 而不直接产生 ATP。
4. Lactate Fermentation in Animals | 动物中的乳酸发酵
In mammalian muscle cells during strenuous exercise, oxygen supply cannot meet demand. Pyruvate accepts hydrogen atoms from reduced NAD and is reduced to lactate (lactate ion). The enzyme lactate dehydrogenase catalyses this reaction.
在哺乳动物肌肉细胞剧烈运动期间，氧气供应无法满足需求。丙酮酸接受还原型 NAD 中的氢原子，被还原为乳酸（乳酸根离子）。乳酸脱氢酶催化该反应。
CH₃COCOO⁻ + NADH + H⁺ ⇌ CH₃CH(OH)COO⁻ + NAD⁺
This reaction is reversible. It allows NAD⁺ to be regenerated, so glycolysis can continue producing 2 ATP per glucose even when oxygen is limited. However, lactate accumulation lowers pH and contributes to muscle fatigue.
该反应是可逆的。它使 NAD⁺ 得以再生，因此即使在氧气有限时，糖酵解也能继续每分子葡萄糖产生 2 个 ATP。然而，乳酸积累会降低 pH 值并导致肌肉疲劳。
5. Alcoholic Fermentation in Yeast and Plants | 酵母和植物中的酒精发酵
Yeast and some plant tissues use alcoholic fermentation. First, pyruvate is decarboxylated by pyruvate decarboxylase to form ethanal (acetaldehyde), releasing carbon dioxide. Then alcohol dehydrogenase reduces ethanal to ethanol using reduced NAD.
酵母和某些植物组织利用酒精发酵。首先，丙酮酸在丙酮酸脱羧酶作用下脱羧生成乙醛，并释放二氧化碳。然后，乙醇脱氢酶利用还原型 NAD 将乙醛还原为乙醇。
CH₃COCOO⁻ + H⁺ → CH₃CHO + CO₂
CH₃CHO + NADH + H⁺ → CH₃CH₂OH + NAD⁺
The overall yield is 2 ethanol, 2 CO₂ and 2 ATP per glucose. The CO₂ makes dough rise and creates bubbles in beer; ethanol is the alcohol in alcoholic drinks.
每分子葡萄糖总产率为 2 个乙醇、2 个 CO₂ 和 2 个 ATP。二氧化碳使面团膨胀并在啤酒中产生气泡；乙醇是酒精饮料中的酒精成分。
6. Energy Yield: ATP Accounting Without Oxygen | 无氧条件下的能量账本
Aerobic respiration releases about 30–32 ATP per glucose in most eukaryotic textbook calculations, because NADH and FADH₂ feed electrons into the electron transport chain and drive chemiosmosis. Anaerobic fermentation yields only the 2 net ATP from glycolysis.
在大多数真核生物教科书计算中，有氧呼吸每分子葡萄糖约产生 30–32 个 ATP，因为 NADH 和 FADH₂ 将电子送入电子传递链并驱动化学渗透。无氧发酵仅产生糖酵解的 2 个净 ATP。
| Feature | Aerobic respiration | Anaerobic fermentation |
|---|---|---|
